The present invention relates to an auto-iris method and apparatus which are suitable to a video camera.
In general, a video camera for domestic use provides an auto-iris mechanism as shown in JP-A-56-19274, for example. This mechanism serves to automatically change an iris depending on quantity of incident light so that it can constantly keep an amplitude of a resulting video signal constant against change of brightness of an object to be imaged and the background of the object. Now, the description will be directed to the conventional auto-iris mechanism with reference to FIG. 1, in which 1 is a lens, 2 is an iris, 3 is an imaging element, 4 is a signal processing circuit, 5 is an output terminal, 6 is an image detecting circuit, 7 is a level comparing circuit, 8 is a reference voltage source, and 9 is an iris driving motor.
A ray of light is reflected on an object and is incident to the lens 1 and the iris 2. Then, the ray of light is applied to the imaging element 3 on which the image of the object is formed. The imaging element 3 picks up the formed image and generates an electric signal corresponding to the image. The electric signal is sent to the signal processing circuit 4 in which the video signal corresponding to the image is generated. Then, the video signal is supplied as an imaging output at the output terminal 5.
In the process for generating the video signal in the signal processing circuit 4, a luminance signal is supplied to the image detecting circuit 6. The image detecting circuit 6 serves to weight the luminance signal at each spot on a screen, detect an amplitude of the luminance signal, and output a detected voltage V1 having a level corresponding to the amplitude. The level comparing circuit 7 serves to compare the level of the detected voltage V1 with the level of a constant reference voltage V.sub.ref fed from the reference voltage source 8 and control the iris driving motor 9 depending on the compared result so that the iris 2 may be adjusted so as to match the detected voltage V1 to the reference voltage V.sub.ref. It results in keeping the amplitude of the video signal output at the output terminal 5 constant independently of the brightness of the object and the background thereof.
As set forth above, the conventional auto-iris mechanism serves to keep the amplitude of a video signal obtained from an object constant however variable the brightness of the object and the background thereof are. It was the essential function. This function, however, may bring about the following disadvantages.
(1) Considering that an object to be images is blue sky on the overall screen and another object is lawn on the overall screen, the amplitudes of the video signals for both objects are kept constant, though the blue sky is brighter than the lawn. The auto-iris operation is done to keep the brightness of a regenerative screen equal in each of the objects. Hence, the blue sky on the regenerative screen is dimmer than the actual blue sky and the lawn on the regenerative screen is far more luminous than the actual sky, resulting in inclining the color of the lawn to white. It is thus impossible to represent the difference of brightness between the objects on the regenerative screen. PA1 (2) The brightness of the background of an object is different at respective times such as early morning, midday, and evening and on the weather conditions such as fine, cloudy, and rainy ones. The foregoing conventional auto-iris mechanism, however, keeps the brightness of the regenerative screens equal in respective backgrounds, resulting in disallowing the difference of the brightness between respective backgrounds to appear on the regenerative screen.